Not Applicable.
Not Applicable.
This invention relates to an electronic current conversion or measurement technique, specifically to a method to use optical intensity modification within an operational amplifier circuit""s feedback loop to create gain.
Current to voltage converter circuits transform an input current level into an output voltage level. Many different types of electronic systems are constructed with current to voltage converters as major subcomponents. Current to voltage converters are discussed in most basic electronic textbooks because of their fundamental usefulness.
The most often implemented current to voltage conversion circuit, when a virtual ground is desired at the input node, uses an operational amplifier circuit with the positive input node connected to ground. A feedback element, normally a resistor when linear response is desired, is connected to the operational amplifier""s output and feeds current from the voltage output of the operational amplifier back to the negative input node of the operational amplifier. The input current to be converted to a voltage is connected to the negative input node of the operational amplifier. The output voltage for an operational amplifier, operating within the normal linear range, will be such that the current flowing into the negative input node will be matched in magnitude but opposite in polarity from the current flowing into the negative input node from the feedback element. If a linear feedback element is used, like a resistor, then the output voltage is equal to the negation of the input current times the ohmage of the resistor. When increased voltage output is desired for an arbitrary input current level, the value of the feedback resistor is increased. As the value of the feedback resistor is increased some real world problems are encountered. High value resistors are very expensive to make stable with respect to time, temperature, and humidity. The feedback resistor adds significant noise to the output when compared to low noise input sources, for example photodiodes used to measure light intensity levels. The high resistance of the feedback resistor combined with the capacitance of many sensors, like a large area photodiode, result in a limited frequency response. A feedback capacitor must be added in parallel with the feedback resistor to maintain the operational amplifier""s phase stability, further limiting the frequency response of the system.
Various circuits have been proposed to reduce problems caused by using high ohmage resistors for the feedback element including resistor networks created to reduce the size of any single element, and adding output buffer amplifiers to the design. These circuits have various undesirable side effects including increased offset, noise, and bandwidth tradeoffs, which are extensively documented in the prior art.
The use of optical feedback in electronic circuits is not new. U.S. Pat. No. 4,074,143 to Rokos (1978) discloses light emitting and light sensitive devices coupled together optically to create feedback. The disclosed information is related to the creation of positive feedback, which is not desirable within the present invention.
Other inventors have used optical feedback within an operational amplifier circuit. U.S. Pat. Nos. 5,189,307 and 5,283,441 to Fabian (1993, 1994) disclose the use of negative optical feedback within an operational amplifier circuit creating a system with unity gain to obtain electrical isolation. The disclosed information is instructional on obtaining a more precise unity gain system, which is not the goal of the present invention.
The use of optics to transform an input current into an output voltage proportional to the input current, which is the object of the present invention, is not unique. U.S. Pat. Nos. 4,652,764 and 4,752,693 to Nagano (1987, 1988) disclose such schemes. These disclosures do not indicate the use of an optical intensity modification within a feedback system to achieve gain. The gain of the current to voltage response of the system disclosed in U.S. Pat. No. 4,652,764 relies on the gain of transistor elements. The gain of a current ratio circuit, disclosed in U.S. Pat. No. 4,752,693 FIG. 6, relies on setting the relative intensity of two separate light emitting devices controlled by the relative current flowing though the devices. The gain of a second current ratio circuit, disclosed in U.S. Pat. No. 4,752,693 FIG. 8, would be created by selecting a reduced number of photo couplers within the input system compared to the number of photo couplers within the output system.
In accordance with the present invention, a current to voltage conversion system is constructed comprised of an operational amplifier, where the operational amplifier obtains a feedback current from an optical detector which is equal to and opposite in magnitude to a single polarity input current. The output of the operational amplifier controls the intensity of a light source. The optical energy from the light source is transmitted through an optical intensity reduction mechanism, to generate a feedback current related to the illumination of the optical detector. A separate light intensity measurement system is constructed to measure the output of the light source directly, bypassing the effects of the optical intensity reduction mechanism. By measuring the light source directly, the magnitude of the input current can be determined. The increased illumination created by using an optical reduction mechanism within the operational amplifier""s feedback loop allows the separate light source intensity measurement system to be easily constructed using conventional technologies.
In accordance with the present invention, the single polarity current to voltage conversion system is extended to convert bipolar input current to either single polarity or bipolar output voltage by adding additional light sources, optical reduction mechanisms and optical detectors to the single polarity system.
In accordance with the present invention, the bipolar input system is extended to use light intensity to offset the input signal by adding additional light sources. This extension reduces the systems noise and increases measurement stability when compared to using high ohmage resistors at the input of the operation amplifiers to change the offset. The high ohmage resistors add thermal and excess noise and creating resistors which are stable with temperature changes is very difficult.
Accordingly, besides the objects and advantages of constructing a current to voltage conversion scheme using optical feedback described in my above patent, several objects and advantages of the present invention are:
(a) to provide increased signal to noise performance;
(b) to provide a more stable circuit gain with respect to time, temperature and humidity;
(c) to provide faster response without operational amplifier phase stability problems; and
(d) to provide a circuit that is simple to implement with reduced current leakage problems.
Still further objects and advantages will be apparent from a consideration of the ensuing description and drawings.